The rapidity of modern-day date processing imposes severe demands on the ability to produce a printout record at very high speed. Impact printing, in which permanently shaped character elements physically contact a recording medium, are proving to be too slow and too bulky for many applications. Thus, the industry has turned to other alternatives involving non-impact printing schemes using various techniques to cause a desired character to be formed on the recording medium. Some of these involve the use of electrostatic or magnetic fields to control the deposition of a visible characterforming substance, either solid (i.e., dry powder) or liquid (i.e., ink) on the medium which is usually paper. Other systems utilize electrophotographic or ionic systems in which an electron or ion beam impinges on the medium and causes a change in coloration at the point of impingement. Still another system employs a thermal image to achieve the desired shape coloration change. Of more recent import is a printing technique, called ink jet printing, in which tiny droplets of ink are electronically caused to impinge on a recording medium to form any selected character at any location at very high speed. Ink jet printing is a non-contact system which requires no specially treated recording media, ordinary plain paper being suitable, and which requires no vacuum equipment or bulky mechanisms. The present invention relates to this kind of printing system.
Ink jet systems may be classified as follows: (1) continuous, in which ink droplets are continuously spewed out from a nozzle at a constant rate under constant ink pressure; (2) electrostatic, in which an electrically charged ink jet is impelled by controllable electrostatic fields; and (3) impulse, or ink-on-demand, in which ink droplets are impelled on demand from a nozzle by mechanical force or thermal energy. The invention is concerned with a nozzle head for this latter type of system.
Typical of the ink-on-demand system is the approach set forth in U.S. Pat. No. 3,832,579 entitled PULSED DROPLET EJECTING SYSTEM. Here a cylindrical piezoelectric transducer is tightly bound to the outer surface of a cylindrical nozzle. Ink is delivered to the nozzle by means of a hose connected between one end of the nozzle and an ink reservoir. As the piezoelectric transducer receives an electrical impulse, it squeezes the nozzle which in turn generates a pressure wave resulting in the acceleration of the ink toward both ends of the nozzle. An ink droplet is formed when the ink pressure wave exceeds the surface tension of the meniscus at the orifice on the small end of the nozzle.
Another type of ink-on-demand printing is described in U.S. Pat. No. 3,179,042 entitled SUDDEN STEAM PRINTER. This system utilizes a number of ink-containing tubes, electric current being passed through the ink itself. Because of the high resistance of the ink, it is heated so that a portion thereof is vaporized in the tubes causing ink and ink vapor to be expelled from the tubes.
In a cop-pending application, Ser. No. 412,290 filed Sept. 7, 1982 and entitled THERMAL INK JET PRINTER by John L. Vaught et al., and assigned to the instant assignee, an ink-on-demand printing system is described which utilizes an ink-containing capillary having an orifice from which ink is ejected. Located closely adjacent to this orifice is an ink-heating element which may be a resistor located either within or adjacent to the capillary. Upon the application of a suitable current to the resistor, it is rapidly heated. A significant amount of thermal energy is transferred to the ink resulting in vaporization of a small portion of the ink adjacent the orifice and producing a bubble in the capillary. The formation of this bubble in turn creates a pressure wave which propels a single ink droplet from the orifice onto a nearby writing surface or recording medium. By properly selecting the location of the ink-heating element with respect to the orifice and with careful control of the energy transfer from the heating element to the ink, the ink bubble will quickly collapse on or near the ink-heating element before any vapor escapes from the orifice.
It will be appreciated that the lifetime of such thermal ink jet printers is dependent, among other things, upon resistor lifetime. It has been found that a majority of resistor failures is due to cavitation damage which occurs during bubble collapse. Hence it is desirable that resistor wear due to cavitation damage should be minimized as much as possible. In co-pending application Ser. No. 443,711 entitled THERMAL INK JET PRINTER UTILIZING A PRINTHEAD RESISTOR HAVING A CENTRAL COLD SPOT, filed Nov. 22, 1982 by John D. Meyer and assigned to the instant assignee, the resistive element is provided with a central "cold" spot formed of a conductive material, it being assumed that most of the bubble damage occurs at or near the center of the resistor. The cold spot causes the formation of a toroidal bubble which upon collapse is randomly distributed across the resistor surface instead of being concentrated in a small central area of the resistor.
In another co-pending application Ser. No. 449,820 entitled THERMAL INK JET PRINTER UTILIZING SECONDARY INK VAPORIZATION, filed on Dec. 15, 1982 by John D. Meyer and assigned to the instant assignee, still another solution to reducing resistor wear is described. Here the resistive layer is covered with a passivation layer to provide chemical and mechanical protection during operation. The passivation layer in this application may be a thin layer of such materials as silicon carbide, silicon oxide, or aluminum oxide. In co-pending application Ser. No. 443,972 entitled MONOLITHIC INK JET ORIFICE PLATE/RESISTOR COMBINATION filed by Frank L. Cloutier, et al., and assigned to the instant assignee, it is suggested that the passivating or protective layer may be formed of such materials as silicon oxynitride, aluminum oxide or titanium dioxide as well as silicon dioxide. In both of these latter two proposals it will be noted that the protective or passivation layer is formed of a single layer of one distinct material. While these materials, particularly silicon carbide, have been satisfactory as far as their wear properties are concerned, they have one weakness, namely, poor adherence to the underlying metallization.